In the context of the production of stacked structures that are formed, in particular thin layers carried by a support (for example a substrate), it has already been proposed to transfer a thin layer onto the substrate by means of a method that comprises the following main steps:
the formation of a weakened zone at a given depth in a substrate consisting of the material that is to form the thin layer, for example by implantation of a gas at that depth;
the bonding of the implanted substrate (referred to as the donor substrate) onto the support, for example by molecular bonding; and
the separation of the donor substrate amputated from the thin layer (situated between the weakened zone and the initial surface of the donor substrate) and the support, which then carries the thin layer, by fracture (generally during a heat treatment step, usually between 200° C. and 600° C.) in the previously weakened zone.
This kind of solution is described in French patent application No. FR 2 681 472, for example; it is used, for example, to deposit a thin layer of silicon onto a support consisting of a silicon substrate covered with a thin layer of insulative silicon oxide (SiO2) in order to obtain an SOI (Silicon-On-Insulator) type structure.
Although the method described briefly hereinabove can be applied as such in the situation that has just been referred to, certain problems can arise in the conventional application of this method in different contexts, for example if the donor substrate and the support have very different mechanical characteristics.
This is the case in particular if it is required to replace the thin layer of silicon with a thin layer of germanium (Ge) that has certain advantageous electronic properties (such as the mobility of the electrical carriers, which improves the performance of circuits produced on germanium).
The production of this kind of structure (referred to as GeOI, standing for Germanium-On-Insulator) by means of the method previously referred to is, for example, the subject of the paper “Germanium-On-Insulator (GeOI) Structure Realized by the Smart Cut™ Technology”, F. Letertre et al., in MRS proceedings, 809 B4.4 (2004).
In this instance, the conventional application of the thin layer transfer method referred to hereinabove leads to bonding a silicon substrate onto a substrate implanted with germanium, with a view to their separation, in the zone weakened by implantation, by heat treatment. This solution is problematic, however, because of the large difference between the coefficients of thermal expansion of the two materials used (2.6×10−6/° C. for silicon and 5.8×10−6/° C. for germanium). The sudden releasing, at the moment of fracture, of the stresses stored in the structure can cause one or even both substrates to break.
The paper referred to above also proposes carrying out implantation in a layer of germanium, the thickness of which can vary from one micron to a few microns and which is formed epitaxially on the surface of a standard silicon substrate (750 μm thick). The structure subjected to the separation heat treatment therefore behaves as a homostructure because of the small thickness of the germanium compared to the thickness of the two silicon substrates.
This latter solution is nevertheless less advantageous from the electronic point of view because of the high number of dislocations and greater roughness in the epitaxial germanium.
Another known solution for producing a structure including a layer of a first material on a substrate of a second material is, after assembly of a substrate of the first material with the substrate of the second material, to carry out chemical-mechanical thinning of the substrate in the first material. However, this technique cannot be used to obtain layers with a thickness of the order of one micron with a thickness of good homogeneity. Using this technique, the greater the thinning, the less homogeneous the thickness of the residual layer.